One of the many ways of reducing the weight of gas turbine engines is to provide engines with hollow blades, for example hollow fan blades, rather than conventional solid material blades. This can provide a significant reduction in the individual and collective weight of the fan blades, and hence the propulsive fan. A reduction in the weight of gas turbine engines is particularly advantageous when they are intended for use in aircraft installations. Hollow blades are, however, disadvantageous when compared to solid material blades.